Project: sc090025/R1

 

Figure 3.1: Typical operating ranges for different pump types 

---

7 
3.2 Archimedean screw pumps 
Screw pumps are based on the Archimedes principle of a rotating shaft upon which a helical 
blade (or series of blades) fit inside a specially formed channel. As the shaft rotates, a cavity 
moves along the length of the shaft and any water trapped within the cavity moves as well. 
These are commonly found where low heads are required, typically less than ten metres.
Figure 3.2: Typical Archimedes screw pump 
The design is robust and can be found around the world where the reliability inherent with slow 
moving machinery and simple technology is important. The design is not without drawbacks, 
however; the main disadvantage of the screw pump concept is that it is difficult to increase the 
pumping head without considerable structural and mechanical modification, whereas this is 
easy with other types of pump. Also, since the design is dependent upon minimal leakage from 
between the flights and the channel, any wear over time significantly reduces efficiency. The 
screw pump efficiency also depends on the level of incoming flow and if this is below 
recommended levels the efficiency of the station is greatly reduced, as shown in Figure 3.3. 
This may conflict with the requirement to maintain channels at the lowest possible level to 
maximise storage capacity within the system. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3.3: Typical screw pump efficiency graph 

---

8 
The bottom bearing has traditionally been a common failure point, which in turn results in wear 
of the main flights. As bearing technology has improved over the years, the bottom bearing has 
become less problematic but many older designs still employ a completely submerged bearing 
which is very difficult to access and hence maintain.